Particles with a diameter of 100 nm or less are generally called nanoparticles, and are just beginning to be used in various fields because they have properties different from those of general bulk materials of even the same material. Various methods for measuring particle size have been known including the laser diffraction/scattering method. Among them, methods based on the so-called dynamic scattering method (photon correlation method) have been employed mainly for nanoparticles with a diameter of 100 nm or less (refer to Patent Literatures 1 and 2 for example).
The dynamic scattering method utilizes the Brownian motion of particles, including exposing particles performing a Brownian motion in a medium to a light beam, measuring the intensity of scattered light from the particles at a predetermined position, and capturing the fluctuation of the scattered light intensity caused by the Brownian motion of the particles, that is, the temporal change of the scattered light. That is, the method utilizes the fact that to-be-measured particles each perform a Brownian motion with intensity according to its particle size to thereby calculate the particle size distribution of the particles.
However, in the dynamic scattering method (photon correlation method), in which the fluctuation of scattered light from particles is measured, it is necessary to measure a small fluctuation in intense scattered light, that is, to measure the fluctuation of light intensity in a bright field of view. Due to its principle, the problems of poor measurement sensitivity as well as poor S/N cannot be avoided.
As a powerful approach for solving such unavoidable problems in the dynamic scattering method, there has been proposed a method and apparatus for electrophoresing particles dispersed movably in a medium by applying a spatially periodic electric field to the particles, generating a quasi diffraction grating by making the particles have a spatially periodic alteration in concentration, in this state detecting diffracted light obtained by exposing the particles to a laser beam, and thereby calculating the diffusion coefficient and size of the particles from the temporal change of the diffracted light after stopping the application of the electric field (refer to Patent Literature 3).
This proposed method and apparatus utilizes dielectrophoresis or electrophoresis of particles in a medium, including generating a diffraction grating resulting from the concentration distribution (density distribution) of the particles by applying an electric field and, in this state, annihilating the diffraction grating by stopping the application of the electric field, that is, utilizes the fact that the annihilation process depends on the diffusion coefficient of the particles. The diffusion coefficient and therefore the size of the particles can be calculated from the time required for dissipation of diffracted light from the diffraction grating generated from the concentration distribution of the particles.
The apparatus has a specific configuration to apply a spatially periodic electric field to particles dispersed movably in a medium, in which a sample having particles dispersed movably in a medium is housed in a transparent cell, a comb-like electrode pair is formed on the inner surface of portions of wall bodies forming the cell, an AC or DC voltage is made applicable to the electrode pair, a parallel light flux such as a laser beam is applied externally to the portion where the electrode pair is formed in the cell 1, and a detection optical system is provided and adapted to detect diffracted light from a diffraction grating formed through the density distribution of the particles in a specified direction.
In accordance with this proposed method and apparatus, the intensity of diffracted light from the diffraction grating resulting from the concentration distribution of particles is detected, and thus the intensity is greater than that of scattered light from particles obtained in the dynamic scattering method, thereby a more intense signal is to be measured, resulting in a significant improvement in S/N and sensitivity relative to the dynamic scattering method.
Patent Literature 1: U.S. Pat. No. 5,094,532
Patent Literature 2: Japanese Patent Laid-Open Publication No. 2001-159595
Patent Literature 3: Japanese Patent Laid-Open Publication No. 2006-84207